The subject matter of the present invention relates generally to hearing aid apparatus and in particular to direct contact hearing aid apparatus mounted in the ear canal with an output transducer having its output coupling element supported to provide direct electromechanical coupling to the ossicles through the tympanic membrane. The coupling element may engage a contact element mounted on the outer surface of the tympanic membrane, and such elements may be a magnet and a magnetic member to provide a magnetic connection. The invention also relates to hearing aid apparatus employing a piezoelectric plastic film as the output transducer and to a magnetic insertion means for insertion and removal of the hearing aid into the ear canal by magnetic engagement with a holder member on the housing of such apparatus. Externally actuated magnetic switch means are provided within the housing for remote mechanical switching of the electrical connections of a battery and volume control means therein.
The present invention is especially useful as a direct contact hearing aid mounted deep within the ear canal of persons who wish to conceal such hearing aid from the view of others. However, such invention is also useful for other types of external hearing aid having its output transducer external to the tympanic membrane, rather than implanted in the middle ear.
Previously it has been proposed to provide a surgically implantable hearing aid employing a piezoelectric output transducer as shown in U.S. Pat. No. 3,712,962 of Epley issued Jan. 23, 1973 and U.S. Pat. No. 3,764,748 of Branch issued Oct. 9, 1973 or an electromagnetic output transducer as shown in U.S. Pat. No. 3,870,832 of Fredrickson issued May 11, 1975 implanted in the middle ear. This has the disadvantage that it requires surgical operations for implantation, repair and change of batteries. If the power supply batteries are not implanted, and are external to the body, they may be connected by electrical contacts extending through the skin to an implanted output transducer, but this is objectionable to many patients.
Conventional external hearing aids in which the input transducer is a microphone and the output transducer is a loudspeaker which modulates an air column between such speaker and the tympanic membrane of the eardrum, have several disadvantages. These disadvantages include acoustic feedback from the loudspeaker to the microphone, inefficient operation resulting in greater power dissipation and frequent battery changes, and distortion of the acoustical output due to a small diameter speaker diaphragm. Previous attempts to overcome acoustical feedback in conventional external hearing aids have included providing an airtight seal in the ear canal in an attempt to acoustically isolate the output transducer loudspeaker deep within the ear canal from the input transducer microphone as shown in U.S. Pat. No. 3,061,689 of McCarrell et al issued Oct. 30, 1962. This has been only partially successful and has not enabled use of maximum gain without feedback. This also has the disadvantage that the airtight mold used as a seal produces an uncomfortable sensation of fullness and increases the perception of internal noises such as one's own voice. In addition, in the McCarrell patent an ear mold containing the microphone, the amplifier and the battery is positioned in the external portion of the ear so that a volume control for such amplifier may be adjusted manually while the hearing aid is in place in the ear. Unfortunately, this requires that the ear mold piece of the hearing aid be located at a position where it can easily be viewed by persons talking to the wearer which is cosmetically objectionable.
Both electromagnetic transducers and piezoelectric transducers are employed as output transducers in McCarrell, but they are not placed in direct contact with the outer surface of the tympanic membrane in the manner of the present invention. Instead, his output transducers are employed as loudspeakers to produce a sound output by vibrating a plastic diaphragm. However, in one embodiment an iron slug is mounted by adhesive directly on the outer surface of the eardrum and spaced away from the electromagnetic transducer core by an air gap whose width would inadvertently vary depending upon the position of the transducer in the ear canal. The width of such air gap is critical to efficiency since it varies with the third power of such width. The appropriate air gap width for maximum efficiency would therefore be very difficult to obtain by positioning the hearing aid in the ear canal and to maintain with any consistency. In addition, the continuous undirectional stress placed on the eardrum membrane by attraction of the magnetic slug toward the electromagnetic pole piece would stress the ossicular chain and tend to weaken and tear the tympanic membrane.
The hearing aid of the present invention eliminates these disadvantages by positioning the output coupling element of the output transducer in direct contact with the outer surface of the tympanic membrane or with a contact element secured to the outer surface of such membrane, thereby eliminating any airspace between the output transducer and the tympanic membrane. As a result, there is direct electromechanical coupling from the output coupling element of the transducer to the ossicle bones through the tympanic membrane without the generation of any discernible sound waves thereby eliminating acoustic feedback, providing a much more efficient operation and greatly reducing distortion. Also, the lack of undirectional stress would prevent damage to the eardrum.
An experimental hearing aid in which a magnet was attached by glue to the outer surface of the tympanic membrane for movement of the magnet by an induced electromagnetic field produced by a coil on the exterior ear is described by Goode et al in "Audition via Electromagnetic Induction" published July 1973 in Arch Otolaryngol, Volume 98, pages 23-26. This hearing aid by employing electromagnetic induction coupling for movement of a magnet attached to the tympanic membrane is not practical because of the large amount of power required. Such article also describes earlier unsuccessful experiments by Wilska who attached small pieces of iron on the tympanic membrane for vibration by a coil and superimposed constant magnetic field of a permanent magnet which are placed over the ear canal, but the strong magnetic attraction apparently stretched or tore the eardrum and caused severe discomfort and pain. Wilska apparently also attached a small electromagnetic coil to the tympanic membrane with similar results except that the coil temperature also caused burning and pain. Unlike the present hearing aid, there was no direct electromechanical coupling of the output transducer of the hearing aid into engagement with the outer surface of the tympanic membrane or with a contact element provided on such membrane with all of its advantages of excellent sound fidelity, low power requirements and no pain, stress or damage to the eardrum during operation of the hearing aid.
It is interesting to note that as recently as September 1982 researchers were still attempting to implant hearing aid output transducers, such as piezoelectric ceramic vibrators, in the middle ear in order to overcome the disadvantages of conventional external hearing aids. In this regard, see the summary of Japanese research and development projects in the article "Implantable Hearing Aid Project" published in Jetro, September 1982, pages 6 to 10, which discloses a similar hearing aid to that shown in one of the embodiments of the earlier discussed U.S. Pat. No. 3,712,962 of Epley.
It has been previously proposed in U.S. Pat. No. 3,832,580 of Yamamuro et al issued Aug. 27, 1974 and U.S. Pat. No. 4,369,391 of Micheron issued Jan. 18, 1983 to provide a piezoelectric plastic transducer in non-hearing aid devices, such as a phonographic pick-up and pressure sensing cables, made of a piezoelectric plastic film including a natural or synthetic high molecular weight polymers and polyvinylidene fluoride. However, such piezoelectric plastic film has not previous been used in a hearing aid. It has been discovered by the present inventor that the low mechanical vibration impedance of piezoelectric plastic film transducers closely matches that of the middle ear so that it is ideal for use as the output transducer of a hearing aid. This discovery has lead the inventor to develop several different types of piezoelectric plastic film output transducers for hearing aids which are shown herein.